The Airmail Act of 1930 changed the method in which the United States
Post Office awarded mail contracts, eliminating competitive bidding. The
Act created a payment structure based on the weight of the load carried
by the aircraft per mile rather than a flat rate per mile regardless of
the weight of the load.

Contracts were awarded based on aircraft payload.
Companies that had large aircrafts were more likely to be awarded contracts
rather than those companies with smaller aircrafts. This Act encouraged
airline carriers to purchase large aircrafts increasing the likelihood
of being awarded airmail contracts. It also stimulated the carriers to
fill space on the aircraft with passengers.